1. Field of the Invention
The present invention relates to an oil-pressure apparatus for accelerating extension or retraction of a rod of an oil-pressure cylinder.
2. Description of the Related Art
An oil-pressure apparatus is used, for example, for a crusher attached to a boom of a power shovel. The crusher has a movable jaw and opens/closes the movable jaw in accordance with extension/retraction of the rod of the oil-pressure cylinder.
In this specification, the oil-pressure cylinder to actuate a working piece such as a movable jaw is referred to as the “actuating cylinder.” The actuating cylinder contains a piston and a rod moving integrally with the piston in a tube. The inside of the tube is partitioned into a rod-side section on the rod side and a bottom-side section on the opposite side with the piston interposed therebetween.
Japanese Patent Application Publication No. 10-266587 (hereinafter referred to as JP 10-266587) discloses an oil-pressure apparatus which closes a movable jaw by extending an actuating cylinder. When the movable jaw is closed with no load applied on the movable jaw, a counter-balance valve 13 is in a closed state. When the piston of the actuating cylinder discharges oil from the rod-side section, the discharged oil is fed into the bottom-side section of the actuating cylinder through a bottom-side line. Also, Oil from a pipe connector B1 is supplied into the bottom-side section, thereby accelerating the extension of the rod. However, the use of this oil-pressure apparatus reduces thrust of the rod during acceleration (FIG. 1). When the movable jaw comes into contact with a target structure, the counter-balance valve 13 is opened to reset the thrust by oil to the original strength (FIG. 2).
The oil-pressure apparatus disclosed in JP 10-266587 accelerates the closing motion of the movable jaw but does not accelerate the opening motion. In order to readily understand the invention of JP 10-266587, it is assumed that the supply/discharge ratio of oil between the bottom-side section and the rod-side section of the actuating cylinder where the piston of the actuating cylinder is moved is 2:1, and oil is supplied at a flow rate of “1” from a pump through a rod-side line or the bottom-side line. When the movable jaw is closed, oil is discharged from the rod-side section. The discharged oil is returned to the bottom-side line and is added to the oil supplied from B1 to the bottom-side section (FIG. 1). Therefore, when the oil at a flow rate “1” is supplied from the pump through the bottom-side line to the bottom-side section, the oil at a flow rate “1” is returned from the rod-side section to the bottom-side line, so that oil at a flow rate “2” is supplied to the bottom-side section, thereby speeding up the closing motion of the movable jaw by two times as compared with the case where the discharged oil is not returned to the rod-side section. Substantially no pressure loss occurs since the oil discharged from the rod-side section is entirely returned to the bottom-side section.
By contrast, when the movable jaw is opened, the oil at a flow rate “1” is supplied from the pump to the rod-side section without any addition. Thus, the opening motion of the movable jaw is not speeded up. Rather, oil at a flow rate “2” is discharged from the bottom-side section to cause a pressure loss in the bottom-side line, which reduces the oil supplied from the rod-side line and thereby decelerates the movement of the piston. Speeding up the closing motion of the movable jaw may be thought to be enough for the crusher but will make no sense if the opening motion of the movable jaw becomes slower, in view of reduction of the crushing operation time on the whole.
An object of the present invention is to provide an oil-pressure apparatus for accelerating extension and retraction of a rod of an actuating cylinder.